1. Field of the Invention
The present invention relates to an apparatus for monitoring air pressure of a plurality of tires, which are mounted on a vehicle. More particularly, it relates to a tire air pressure monitor which is capable of identifying a position where each of the tires is mounted.
2. Description of the Related Art
In conventional arts, there have been suggested the following methods for transmitting a tire air pressure signal, in particular, a signal including an air pressure abnormality, from a transmitting antenna installed on each tire, and for making a notice as to the tire air pressure via a receiving antenna, which is installed in the vicinity of a tire:    (1) receiving antennas are placed respectively in the vicinity of tires to detect from which receiving antenna a signal is received, so as to determine from which tire a signal is transmitted (e.g., see Patent Reference 1);    (2) one receiving antenna is placed in the vicinity of a plurality of tires, transmission antennas are provided facing to the respective tires, an ID is given by tire, and the tire is identified by confirming the ID via a two-way communication between the transmitting side and the receiving side; and    (3) one receiving antenna is placed in the vicinity of a plurality of tires, a signal from a transmitting antenna for each tire is provided with an ID by tire, and a tire is identified based on the ID in a received signal via one-way communication from the transmission side (e.g., see Patent Reference 2).[Patent Reference 1]
Japanese Registered Utility Model No. 2538986
[Patent Reference 2]
U.S. Pat. No. 5,924,055
According to the method as described in (1) above, it is necessary to install receiving antennas for the respective tires, and it is also required to determine from which receiving antenna a signal is received at the receiving side, thereby making the air pressure monitor more complex. According to the method as described in (2) above, a configuration for carrying out the two-way communication (e.g., a transmitting antenna facing to each tire) is necessary, thereby making the system more complex. Further, according to the method as described in (3), exchanging tires in rotation may cause an inconvenience such that it becomes impossible to determine which tire is mounted in which position.